1. Field of the Invention
The present invention relates to a heat exchanger and, more particularly, to a heat exchanger for condensing vapor containing non-condensable gas and to a power generating plant as well as an absorption refrigerator which incorporates such a heat exchanger.
2. Description of the Prior Art
Condensers are used as heat exchangers for condensing or liquefying vapor such as steam contained in a gaseous mixture which is exhausted from, for example, a steam turbine of a nuclear or ordinary thermal power generating plant and which also contains non-condensable gases such as air.
In general, a condenser has a tube nest composed of a plurality of cooling tubes. In order to improve heat transfer performance of the condenser, it is absolutely necessary to reduce pressure loss of the gaseous mixture posed by the cooling tubes and to remove non-condensable gas. Reduction in the pressure loss enables the gaseous mixture to reach a core region of the tube nest so that the cooling tubes in the core region also can effectively contribute to the condensation. Non-condensable gas such as air, when introduced into the condenser, forms a film or layer on cooling tubes to impede condensation. Removal of such non-condensable gas, therefore, improves condensation of the steam.
Japanese Unexamined Patent Publication No. 61-114087 proposes a condenser which is improved to reduce pressure loss while facilitating removal of non-condensable gas. This condenser has a tube nest which has radially outer portion in which tube groups and steam access lanes are arranged alternately and a radially inner core region in which tube groups are arranged densely. A non-condensable gas discharge port is provided in the core portion of the core region where tube groups are arranged densely. In this condenser, the gaseous mixture containing steam and non-condensable gases is introduced through the steam access lanes in the radially outer region so that the pressure drop is reduced as compared with known condensers in which tubes are uniformly arranged at high density. The core region having high density of heat transfer tubes effectively liquefies the steam and gases which could not be condensed are discharged through the non-condensable gas discharging port.
The condenser disclosed in Japanese Unexamined Patent Publication No. 61-114087 is constructed on an assumption that the gaseous mixture including steam flows only along the steam access lanes. In order to condense the steam which could not be condensed on the tubes in the radially outer region, all the steam passages are led to the central core region where the cooling tubes are arranged densely. Consequently, the following two problems are encountered with this type of condenser.
Firstly, it is to be pointed out that the non-condensable gases tend to be led to low-pressure portions which are formed in the condenser due to specific patterns of streams of the gaseous mixture and unevenness or local variation of the condensation taking place in the condenser. Thus, a considerable portion of the non-condensable gases fails to flow into the core region of the condenser. This makes it difficult to efficiently discharge non-condensable gases, resulting in stagnation of such gases within the condenser.
The second problem is that the pressure loss is large because of the highly dense arrangement of cooling tubes in the core region arranged to occupy a large area facing the outlet ends of the steam access lanes. This pressure loss lowers the temperature of saturated steam, which in turn lowers the heat transfer.